The UBL21 is one of very few double diode audio output pentode valves that were designed. This exhibit has lost all marking but comparison with EBL21 gave a positive identification. The heater only drew 40 mA at 6.3 Volts. At 50 volts it took 100 mA - exactly the requirement of the UBL21.
The valve has a common cathode and the diodes are for signal detection and AGC rectification. A large signal was required for good output and sets using these valves were designed for local signal reception in a domestic setting.
The small plate to the left of the central cathode and below the lower mica is one of the diode anodes. The other is on the other side of the cathode.
The diode anodes. In the main section to the right of the image is the peened notch containing part of the suppressor grid winding.
The wide glass tube envelope is 28 mm in diameter and, excluding the B8B base pins, is 78 mm tall.
References: Datasheet, 1040 & 3002. Type UBL21 was first introduced in 1941.